Molecular vibrations have oscillation periods that reflect the molecular structure, and may hence be used for detection and unique identification of molecules. Particularly, vibrational spectroscopy allows unique identification of molecules having high environmental and biological impact, and may therefore find application in microscopy without fluorescent markers and to environmental and biomedical diagnostics.
Among the various vibrational spectroscopy techniques, Coherent Anti-Stoke Raman Scattering (CARS) is a powerful technique for acquiring chemically selective images of biological and chemical specimens. CARS is based on the Raman effect and uses two laser beams, a pump beam at a center frequency ωp and a Stokes beam at a center frequency ωs. The optical process is a nonlinear third-order process: a pump photon and a Stokes photon interact with a specimen and excite the vibrational level at frequency Ωvib=(ωp−ωs). A third photon, at frequency ωp, also coming from the pump beam, interacts with the excited vibrational level and stimulates the emission of a coherent photon at anti-Stokes frequency ωas=(2ωp−ωs). As a result, when Ωvib is resonantly tuned with a given vibrational mode associated with a molecule, a CARS signal of anti-Stokes frequency ωas is obtained, which may be utilized for high-sensitivity detection of molecules characterized by the vibrational frequency Ωvib. The CARS technique may be used in both microscopy and spectroscopy, to obtain images of in vitro and in vivo cell structures, in sensor technology or for detection of molecules having a high environmental impact.
In spite of its high potential, the CARS technique still suffers from certain implementation difficulties. Since CARS is a nonlinear third-order process, high peak powers, which can be obtained with short light pulses, are necessary to obtain significant efficiencies. On the other hand, there exists a lower limit for pulse duration: since Raman transitions have typical line widths of the order of a few cm−1, pump and Stokes pulses of narrow bandwidth are necessary, which correspond to minimal durations of a few picoseconds for efficiently exciting the system and allowing discrimination of close vibrational frequencies. Practical applications of the CARS technique generally require two or more synchronized laser sources emitting picosecond pulses, with a frequency difference tunable in a broad range. E. O. Potma and Sunney Xie in “CARS Microscopy for Biology and Medicine”, published in Optics & Photonics News, November 2004, pages 40-45, gives a brief overview of the evolution of certain CARS schemes.
The paper “High-sensitivity coherent anti-Stokes Raman scattering microscopy with two tightly synchronized picosecond lasers” by E. O. Potma et al., published in Optics Letters, vol. 27, No. 13 (2002), pages 1168-1170, describes a CARS scheme that uses two picosecond Ti:sapphire lasers synchronized via a phase-locked-loop. The CARS image is formed by a sample scan realized by piezo transducers.
In “An Epi-Detected Coherent Anti-Stokes Raman Scattering (E-CARS) Microscope with High Spectral Resolution and High Sensitivity” by Ji-Xin Cheng et al., published in The Journal of Chemical Chemistry B, vol. 105, No. 7 (2001), pages 1277-1280, CARS detection is carried out using two picosecond Ti:sapphire laser sources synchronized via a “lock-to-clock” system.
Patent application US 2008/0037595 discloses a system for generating an electromagnetic pump field at a first frequency and a Stokes field at a second frequency for a CARS system. The system includes an optical parametric oscillator based on a periodically poled nonlinear crystal which is synchronously pumped by the second harmonic output of a picosecond laser to obtain a pulsed signal at a signal frequency (providing the pump field) and a pulsed signal at an idler frequency (providing the Stokes field). A tuning system adjusts the temperature of the nonlinear crystal to change the difference between the signal frequency and the idler frequency.
Some authors have suggested the use of a single ultra-short (femtosecond) pulse for the whole CARS process. The paper “Single-pulse coherently controlled nonlinear Raman spectroscopy and microscopy”, by N. Dudovich et al., published in Letters to Nature, vol. 418 (2002), pages 512-514, discloses an experimental CARS spectroscopy system consisting in a mode-locked Ti-sapphire laser, which emits 20 fs pulses at 80 MHz, a programmable pulse shaper (an electronically controllable spatial light modulator) and a photodetector. Spectral data is obtained by shaping the excitation pulse to resolve the weak resonant signal from the strong non-resonant noise.
A CARS system that uses a single fs source associated with a pulse shaper is also disclosed in WO 2004/068126.
Periodically poled crystals (pp) are frequently used as nonlinear optical materials and are typically more efficient in second harmonic (SH) generation, than crystals of the same material with no periodic structure.
The spectral and temporal properties of a femtosecond pulse may be significantly changed by quadratic interactions under large group velocity mismatch (GVM). The paper “Narrow-bandwidth picosecond pulses by spectral compression of femtosecond pulses in a second-order non linear crystal” by M. Marangoni et al., published in Optics Express, vol. 15, No. 14 (2007), pages 8884-8891, describes a technique for spectral compression of broadband femtosecond pulses based on the nonlinear optical second harmonic (SH) generation process under large GVM between interacting pulses. A high GVM implies a very narrow bandwidth in which the phase matching condition is fulfilled, and therefore a narrow bandwidth generation of picosecond SH pulses. This technique provides a “spectral compression” that can use a broadband fundamental frequency (FF) pulse to generate narrowband SH pulse with high spectral tunability. SH conversion is obtained using a pp stoichiometric lithium tantalate crystal (pp-SLT). Wavelength tuning is obtained by changing the phase-marching condition in the crystal and changing the pump wavelength.
“Designer femtosecond pulse shaping using grating-engineered quasi-phase-matching in lithium niobate” di Kornaszewski et al., published in Optics Letters, vol. 33, No. 4 (2008), pages 378-380, describes the use of an aperiodically poled lithium niobate crystal for generation of femtosecond pulses with fully engineered intensity and phase profiles using second harmonic generation by an erbium-doped optical fiber laser.